1. Field of the Invention
This invention relates to methods and apparatus for belling pipe and more particularly to methods and apparatus for belling plastic pipe.
2. Description of the Prior Art
A particularly convenient way to join one plastic pipe to another is by adhesion, or "glueing". To facilitate this adhesion, one end of the pipe is belled; that is, enlarged just enough to allow an unbelled pipe end to closely fit within the belled section. In order to make a good seal, the belled section must be smooth and uniformly sized to within close tolerances. Further, to prevent breakage upon stress after joining, the belled section must be of a uniform thickness. Finally, these characteristics must be maintained even after storage and handling, so that the plastic in the belled section must be of a proper set or hardness.
In the past, belling has been achieved by heating the ends of the pipe in a furnace or the like for a set period of time, removing them from the furnace, and then forcing the ends over a mandrel. In this manner, the furnace temperature and the period during which the pipes were heated were monitored to achieve the proper pipe temperature for belling. This pipe temperature is crucial since it drastically affects each of the variables mentioned before. For example, if the pipe temperature is not high enough (not beyond its memory temperature) then the set or hardness of the belled plastic will not be enough to prevent its creeping back to its old shape; especially if exposed to sunlight or heat. If the temperature is too high (above its damage or breakdown temperature) the integrity of the plastic is is reduced and it becomes brittle or flaws develop in the surface. Further, if the heating is not uniform throughout the length, circumference, and thickness of the belled section then the above problems can occur in localities or the belling can be off-center. All of these are a problem in the prior art, since the pipe temperature is maintained only by monitoring the surface temperature and the length of time the pipe is in the furnace. These factors can change suddenly if a sudden draft of wind occurs or the removal of the pipe from the furnace is delayed. This is evidenced by a large percentage of defective pipes made in the prior art. These defects can especially be a problem if they are not discovered until they leak or a break occurs after the pipe is installed underground.